Laser ultrasonic receivers based on optical homodyne interferometers have been investigated for some years. One such receiver is disclosed in U.S. Pat. No. 5,900,935 issued May 4, 1999. Such receivers have been used and proposed for the examination of materials, such as, for example, investigating transient body transformations, inspecting materials such as metals and ceramics at high temperatures for process and quality control, detecting flaws as soon as they are created, measuring production parameters such as thickness and temperature, and determining microstructural properties on-line such as grain size, porosity and the like. In early research, it was realized that a classical homodyne interferometer could not operate effectively with the speckled beams that result from reflecting from rough surfaces. Furthermore, such early homodyne interferometers could not compensate for dynamic changes in the signal beam wavefront resulting from slow, environmental disturbances.
Time-delay or self-referencing interferometers have been developed, such as the confocal Fabry-Perot which allow the processing of light scattered from rough surfaces with a large field of view. Usually, a phase modulated signal beam is derived from a probe beam scattered or reflected from a vibrating test surface. This beam is demodulated by the slope of the transfer function, which is the transmission versus frequency, of the confocal Fabry-Perot. As a self-referencing or time-delay interferometer, the confocal Fabry-Perot has the ability to process speckled beams from imperfect surfaces. In addition, the particular mirror curvature of the confocal Fabry-Perot provides a much larger field of view than a Fabry-Perot with flat mirrors. The operation of the confocal Fabry-Perot is described in, for example, U.S. Pat. No. 4,659,224. However, the confocal Fabry-Perot requires stabilization of the interferometer length to a fraction of an optical wavelength, thereby adding complexity and cost to the receiver.
The transmitted signal from a confocal Fabry-Perot is proportional to the amplitude of the Doppler shift of the signal beam frequency upon scattering from a vibrating surface. For constant displacement, the Doppler shift decreases with frequency. As a result, the confocal Fabry-Perot does not work well at ultrasonic frequencies below approximately one megahertz (1 MHz).
Solutions to such problems and limitations have been proposed. See, for example, U.S. Pat. No. 5,131,748 to Monchalin and Ing, where the beam that probes the vibrating surface is caused to interfere inside a photorefractive material with a reference or pump beam, resulting in these two beams diffracting in each other's direction with a common path and a common wavefront. An electrical signal dependent on phase excursions or perturbations in the reflected or scattered beam produced by the surface vibration is then obtained by a photodetector in one of these paths. For the correct static phase difference between the wavefronts of the two interfering beams, the electrical signal is linearly proportional to the phase excursion and thus to the surface displacement. The photorefractive material acts in effect as a real-time hologram providing an exact overlap of the reference beam with the signal beam for later coherent detection and compensation for low frequency dynamic environmental distortions in the signal wavefront. Effective dynamic compensation requires that the response time of the real-time hologram be on the order of 10-100 microseconds while maintaining high diffraction efficiency.
In operation of a homodyne interferometer having an adaptive holographic beam splitter, a coherent, polarized light beam is split, one of the beams being used as a reference beam. The other beam is reflected or scattered from a surface of the material which is vibrated by an ultrasonic frequency source. The reflected beam has its phase shifted in proportion to the surface deflection or perturbation and is impinged on the surface of a multiple quantum well adaptive holographic beam splitter. The reference beam is also impinged onto the surface of the multiple quantum well adaptive holographic beam splitter to create effectively an interference of the two beams, resulting in a refractive index and/or an absorption grating. This grating causes the beams to diffract into each other, so that the original beam and the diffracted beam are co-propagating and have identical wavefronts. The beam with superposed wavefronts is received by a photodetector which senses the high frequency dynamic phase difference between the two beams and produces a signal representative of the perturbations of the vibrating test surface.
Interferometers using homodyne detection with dynamic wavefront compensation differ from classic interferometers in that the final and critical step of combining the signal and reference beams is performed by a photorefractive crystal acting as a real time hologram. The mixing process in this crystal is known as two-wave mixing. There are three benefits to the use of two-wave mixing as a means for generating a real time hologram. First, a plane wave reference beam can be combined with a speckled signal beam and have perfect spatial overlap for mixing at the photodetector. Second, the spatial phase of the photorefractive grating insures that the phases of the two combining beams are exactly in quadrature, which is the requirement for linear detection. The third benefit needs some explanation. The photorefractive grating has a finite response time. For any phase changes in either arm that occur on a slower time scale, the crystal can respond, the spatial grating accommodates to the changes and thus no signal is detected. For any phase changes that happen rapidly compared with the response time (i.e. the ultrasonic signals), the grating is frozen temporally and spatially, and simply acts as a static hologram that combines the two beams. Put in another way, the crystal acts as a high-pass filter, with a cutoff frequency determined roughly by the inverse of the response time. Slow perturbations are compensated, while fast ultrasonic phase changes are optimally detected. This high-pass property is a major benefit. In a real application it is desirable to reject all (low frequency) mechanical perturbations (jamming signals), while optimally detecting the desired ultrasonic signals. The effectiveness of the filtering depends largely on the value of the cutoff frequency. Ideally, it should be 1-10 kHz for good jamming rejection. The frequency response should also drop as fast as possible below the cutoff frequency.
For a given wavelength of operation, certain crystals are preferred, based on their wavelength response. Although applicable to all II-VI semiconductor crystals, at the wavelength of 1550 nm, one of the best crystals is CdTe doped with V or Ge. These crystals are characterized by a low frequency tail in their frequency response that is undesirable and which is not predicted from a basic model and not seen in other types of crystals. It is desired that the response at low frequencies not only be small, but in fact be much smaller (by orders of magnitude) than the response at high frequencies.